There are two clinically important uses for the auditory middle latency response (MLR) in patients with communicative disorders: (l) the identification of lesions along the auditory pathway central to the brainstem, and (2) the assessment of low frequency hearing sensitivity. For the MLR to be effective as a diagnostic and prognostic measure, the specific generator sites underlying the response must be determined and the physiologic response properties understood. To these ends, we will employ mapping techniques on the cortical surface, depth recordings of primary and association auditory cortex, neural inactivation techniques (lidocaine injection), and histologic reconstruction. The effects of contralateral white noise and binaural stimulation will be examined. As a measure of low frequency hearing sensitivity, MLRs are most needed for patients for whom behavioral audiometry is impractical. These patients are primarily children. It is becoming increasingly recognized that adult MLRs may differ from the responses obtained from children. We propose to study the effects of maturation on MLRs in an animal model, which lends itself easily to longitudinal study. Response properties (wave morphology, latency, amplitude) and response variability (percent detectability) will be investigated in immature and adult animals. Parameters to be investigated include bandpass filter settings, filter slope, and rate of stimulation. The relation of MLR development to ABR maturation will be studied.